Anchimeric assistance 2- ethyl

Recently, Wiesner et al. [108] succeeded in a stereoselective P-glycosylation in the D-ribo series by use of an anchimeric assistance from the 3 position. The 3-O-p-methoxy-benzoyl group is supposed to form the 1,3-acyloxonium ion intermediate 215 either starting from an at- or P-configurated ethyl thioglycoside precursor (214). 

The reported analyses of the products suggested that the COOCH2CH3 substituent provided anchimeric assistance for the reaction paths 2 and 3. The mechanism was explained in terms of an intramolecular solvation of the bromide ion through an intimate ion-pair intermediate which decomposes in two different directions (equation 74, where X = Br and CH3CH2 replaces CH3). The formation of bromobutyric acid and ethylene (path 3) indicated a normal six-membered transition state for their formation as found in primary ethyl esters pyrolyses in the gas phase. Bromobutyric acid, which is known to be unstable at room temperature, rapidly produced butyrolactone. The consecutive reaction of path 4, under the experimental conditions, was ascribed to a similar mechanism where an intimate ion-pair intermediate is formed through COOH participation (equation 76). 

In connection with the methoxy participation, the gas-phase pyrolytic elimination of 4-chloro-1 -butanol was investigated177. The products are tetrahydrofuran, propene, formaldehyde and HCl. It is implied that the OH group provides anchimeric assistance from the fact that, besides formation of the normal unstable dehydrochlorinated intermediate 3-buten-l-ol, a ring-closed product, tetrahydrofuran, was also obtained. The higher rate of chlorobutanol pyrolysis with respect to chlorethanol and ethyl chloride (Table 27) confirmed the participation of the OH group through a five-membered ring in the transition state. 

The homogeneous unimolecular decomposition of o-hydroxy-2-phenvlethyl chloride yielded mainly benzodihydrofuran, HC1 and much less o-hydroxystyrene. The rate was significantly higher than that of phenylethyl chloride and ethyl chloride (Table 28). According to the nature of the product formation and the kinetic data, the OH provided anchimeric assistance in the elimination process. The mechanism proposed is described in equations 87 and 88. 

Anchimeric Assistance by Alkyl Groups The formolysis of neopentyl tosylate (612, X=OTs) is as rapid as that of ethyl tosylate whereas the rate of tri-fluoroacetoly sis is enhanced by a factor of 160. These data have been taken as evidence... 

The ethylthio group seems to provide more anchimeric assistance than the phenylthio group, as evidenced by the observation that 2-chloro-ethyl ethyl sulfide solvolyses about 40 times faster than 2-chloroethyl phenyl sulfide. This may be ascribed to a decreased nucleophilicity of the arylthio groups by resonance delocalization of the electrons on sulfur into the aryl ring. 

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